Automobile internal combustion engines produce torsional vibrations that are undesirable to transmit through the vehicle transmission. To isolate such torsional vibrations, torsional dampers may be implemented within the vehicle transmission. These dampers typically rest between the engine crankshaft and the input shaft of the transmission to substantially counteract the unwanted torsional vibrations generated by the engine. Dampers are typically configured with compliant members that have the capacity to carry maximum engine torque plus an additional margin. The damper system may employ a damper lock-out clutch to avoid the torsional resonance associated with the starting and stopping of the engine.
One premise behind hybrid automobiles is that alternative power is available to propel the vehicle, thus reliance on the engine for power can be decreased, thereby increasing fuel economy. Since hybrid vehicles can derive their power from sources other than the engine, hybrid vehicles may operate at low engine speeds and the engine may be shut down while the vehicle is propelled by the electric motors. For example, some electrically variable transmissions alternatively rely on two electric motor/generators housed within the transmission to effect movement of the vehicle. Engines in hybrid vehicles are therefore required to start and stop more often than engines in non-hybrid systems. Therefore, greater functionality is desirable in the damper system to allow the lock-out clutch to be actuated in various modes of operation such as engine shut down and start up, as well as while operating in purely electric mode.
Additionally, the stators for each electric motor/generator contained within the electrically variable hybrid transmission may each require differing rates of cooling that are dependent on the duty cycle of each motor/generator. The cooling of the stator is typically performed by bathing the stator with a calibrated flow rate of transmission fluid, thereby allowing the heat generated by operation of the motor/generators to be transferred to the fluid. A continuously high cooling rate is simple to implement, however, additional pump loads and spin losses may produce a decrease in efficiency over a selectively controllable motor/generator cooling system.